1. Field of the Invention:
The invention relates to electric sequencing relays and to heating systems employing electric sequencing relays for sequentially energizing electric heating elements.
2. Description of the Prior Art:
The prior art, as exemplified by U.S. Pat. No. 2,041,775, No. 2,266,537, No. 2,498,127, No. 2,574,869, No. 3,257,526, No. 3,405,380, No. 3,546,652, No. 3,634,801, No. 3,688,060 and No. 3,713,062 contains a number of thermally operated relay devices. Some of the prior art devices employ snap acting elongated bimetal members heated by internal current and upon which contacts are mounted; other devices employ elongated non-snap-acting bimetal elements having a free end engaging an elongated member slidable in housing for operating a switch, such as a snap-acting switch; still other prior art devices employ a plurality of snap-acting discs heated by separate heaters and which engage slidable motion transfer pins engaging flexible contact arms. Heating of bimetal elements, either by internal resistance heating or by heating coils wound on the bimetal elements in thermo-relays results in repeated flexing of leads and/or coils causing failure of such leads and/or coils. Snap acting switches usually employ separate spring devices, or the like, for producing a snap action in closing the switches resulting in higher cost for thermo relays containing such thermo relays. Snap acting discs are generally formed with considerable die force producing appreciable metal flow to provide the disc shape required for snap action, resulting in increased costs and/or less uniformity in the temperature response of the discs. Also, prior art thermo relays using a sliding member in a housing wherein the member is moved in one direction by a resilient contact arm can result in significant reduction of contact force due to friction forces or gravity forces on the sliding member.
The prior art also contains a number of heating systems employing thermally operated relays to sequentially energize heating elements, as exemplified in U.S. Pat. No. 3,046,380, No. 3,242,978, No. 3,329,869, No. 3,351,739, No. 3,588,471, No. 3,659,155 and No. 3,770,977. Generally, air blowers in such heating systems must be maintained in operation when any of the heating elements is energized to prevent burn out or failure of the heating elements. Some of the prior art systems utilize a lower temperature operated thermo-switch device than that employed to energize heating elements, and others employ heater element current sensing facilities for maintaining air blower operation.